Allow machine dce to remove uses in the same instruction

Machine DCE cannot remove a dead definition if there are non-dbg uses.
A use however can be in the same instruction:

  dead %0 = INST %0

Such instructions sometimes created by Detect dead lanes pass.

Allow this instruction to be deleted despite the use if the only use
belongs to the same instruction.

Differential Revision: https://reviews.llvm.org/D59565

llvm-svn: 356619

1 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
index 36c7de67f5d..049ce706330 100644
--- a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
+++ b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
@@ -81,9 +81,11 @@ bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
         if (LivePhysRegs.test(Reg) || MRI->isReserved(Reg))
           return false;
       } else {
-        if (!MRI->use_nodbg_empty(Reg))
-          // This def has a non-debug use. Don't delete the instruction!
-          return false;
+        for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
+          if (&Use != MI)
+            // This def has a non-debug use. Don't delete the instruction!
+            return false;
+        }
       }
     }
   }